1. Field of the Invention
This invention relates to improvements in electronic fuel control systems for internal combustion engines and particularly to improvements in automotive electronic fuel control systems wherein the quantity of fuel injected into the engine is computed from signals indicative of the engine's operating parameters derived from various sources. In particular, the present invention is a fuel control system deriving a signal indicative of the density of the air in the engine's intake manifold from an air density computer. The system accurately computes the engine's fuel requirement maintaining the desired air to fuel ratio during the transient period, between the time the engine is started and the time the engine and its accessories reach normal operating temperatures.
2. Description of the Prior Art
The known electronic fuel control systems employ various sensory inputs to compute the engine's fuel requirements of the internal combustion engine. It is well known that fuel enrichment is required to start a cold engine and sustain its operation until some nominal engine temperature, usually about 25.degree.C is reached. The prior art has recognized the requirement for fuel enrichment during the warm-up period of an internal combustion engine, and a number of patents disclose means for providing additional fuel to the engine during the warm-up period. However, the prior art fuel enrichment has primarily been directed to sustaining engine operation during the warm-up period and is terminated as soon as possible. Recent concern over the emission of excessive undesirable gases from the exhausts of automotive engines has caused attention to be focused on the warm-up period during which many of the engine's operating parameters are changing.
Various factors are known to influence the engine's fuel requirements during the transient period between the time the engine is started and it reaches normal operating temperatures of approximately 75.degree.. The first and the overriding factor is the condensation of fuel injected into the cold air intake manifold. This is a short term process which terminates when the surfaces surrounding the intake valve become sufficiently warm to cause complete vaporization of the injected fuel. The injected fuel is normally sprayed in the immediate vicinity of the intake valve which heats relatively fast, ending the requirement for additional fuel after a predeterminable temperature of about 25.degree.C is reached. This time is normally less than a few minutes.
The second factor is the change in the air density as a function of its temperature. Test have demonstrated that during the transient period between start and time the engine reaches normal operating temperatures carbon monoxide (CO) emissions gradually increase. This increase in CO emissions extends well beyond the time when conventional fuel control computers responding to engine sensors monitoring the temperature of the engine or its coolant have ceased to correct the fuel delivery as a function of engine temperature. The exhaust carbon monoxide level is known to be an indicator of the engine's air to fuel ratio, and since it is assumed that the fuel control computer is correcting properly for the pressure in the air intake manifold, the increase in carbon monoxide level is attributed to changes in the density of the air during this transient period.
The temperature of the air being inhaled by the engine is a function of various factors. The first is the ambient temperature of the air prior to entering the air intake manifold. This factor is discussed in U.S. Pat. No. 3,456,628 which discloses a temperature sensor at the inlet to the air intake manifold providing a second order correction to the fuel control computer for the ambient air temperature. However, prior to entering the engine's combustion chamber, the ambient air is heated by several sources, these are: (1) the air cleaner, throttle body and intake manifold which are in thermal contact with the engine and warm up more slowly due to their thermal inertia and remote location; (2) the recirculated exhaust gases (EGR) which are admitted into the intake manifold to reduce the emissions of the nitrogen oxides (NO.sub.x), (3) the residual exhaust gas left in the cylinder and the intake port after the exhaust stroke of the engine, and (4) the intake port and the interior surfaces of the cylinders which become hotter after the engine warms up. These sources heat the air to different degrees and at various rates which may extend well beyond the time indicated by sensors detecting the engine temperature or the temperature of its coolant.
The prior art has also recognized that fuel requirement of an engine during the warm-up period is a composite of factors other than the temperature at the intake valve port and the temperature of the ambient air. U.S. Pat. No. 3,566,846 touches upon corrections to the quantity of fuel delivered to the engine as being a composite of two phenomenon, one having a short time constant which may be related to the condensation phenomenon discussed above, and the second having a longer time constant (approximately X5) which may be a combination of fuel and air temperatures. The fuel delivery correction of U.S. Pat. No. 3,566,846 is based on information received from an oil temperature sensor having approximately the desired temperature lag. However, in the above patent, the correction is only operative when the oil temperature is below 0.degree.C, and therefore is directed to the starting and initial warm-up period of an engine operating in an extremely cold environment below 0.degree.C and not to the transient conditions which still exists at temperatures well above 0.degree.C. Extending the concept of this patent to cover the desired temperature range provides an approximate correction to the fuel requirements of the engine, but is deficient because the temperature sensed is subject to variables which are not directly related to the temperature of the air in the intake manifold of the engine. The quantity of oil in a conventional engine may vary by as much as 20 percent, the viscosity of the oils used in winter and summer differ considerably, and the contaminated state of the oil may cause significant changes in its thermal properties. These and other independent variables degrade the correspondence between the rate at which the air in the intake manifold heats up and the rate at which the oil heats up.
U.S. Pat. No. 3,589,345 briefly mentions an auxiliary control device of conventional form, to compensate for engine and air temperature and suggests a single sensor at the intake valve port for measuring the combined effect of both engine and air temperature. The function of this single sensor is directed towards the cooling effect of cold air on the temperature of the engine at the intake valve port, i.e., the compensation for the condensation of fuel injected into a cold engine, rather than the change in air density as a function of temperature during the transient warm-up period. The sensor discussed in Pat. No. 3,589,345 is the conventional engine temperature sensor located in a unique position and gives a more accurate determination of the temperature in the area where the fuel is injected than a remote sensor measuring the temperature of the engine's coolant.
Another patent which is also addressed to fuel compensation as a function of temperature is U.S. Pat. No. 3,605,703 which discloses a method for correcting the length of the fuel injection pulse to compensate for the formation of gas bubbles in the fuel when the engine is running hot or after a hot soak. The above patent is directed to the upper limits of the engine temperature spectrum and the density of the fuel rather than the density of the air.